A wide assortment of active implantable medical devices (AIMD) are presently known and in commercial use. Such devices include cardiac pacemakers, cardiac defibrillators, cardioverters, neurostimulators, and other devices for delivering and/or receiving electrical signals to/from a portion of the body. Sensing and/or stimulating leads extend from the associated implantable medical device to a distal tip electrode or electrodes in contact with body tissue.
The hermetic terminal or feedthrough of these implantable devices is considered critical. Hermetic terminals or feedthroughs are generally well-known in the art for connecting electrical signals through the housing or case of an AIMD. For example, in implantable medical devices such as cardiac pacemakers, implantable cardioverter defibrillators, and the like, a hermetic terminal comprises one or more conductive pathways which may include conductive terminal pins, conductive filled vias, leadwires and the like supported by an insulative structure for feedthrough passage from the exterior to the interior of an AIMD electromagnetic shield housing. Hermetic terminals or feedthroughs for AIMDs must be biocompatible as well as resistant to degradation under applied bias current or voltage (biostable). Hermeticity of the feedthrough is imparted by judicious material selection and carefully prescribed manufacturing processing. Sustainable hermeticity of the feedthrough over the lifetime of these implantable devices is critical because the hermetic terminal intentionally isolates the internal circuitry and components of the device (AIMD) from the external environment to which the component is exposed. In particular, the hermetic terminal isolates the internal circuitry, connections, power sources and other components in the device from ingress of body fluids. Ingress of body fluids into an implantable medical device is known to be a contributing factor to device malfunction and may contribute to the compromise or failure of electrical circuitry, connections, power sources and other components within an implantable medical device that are necessary for consistent and reliable device therapy delivery to a patient. Furthermore, ingress of body fluids may compromise an implantable medical device's functionality which may constitute electrical shorting, element or joint corrosion, metal migration or other such harmful consequences affecting consistent and reliable device therapy delivery.
In addition to concerns relative to sustained terminal or feedthrough hermeticity, other potentially compromising conditions must be addressed, particularly when a hermetic terminal or feedthrough is incorporated within an implantable medical device. For example, the hermetic terminal or feedthrough pins are typically connected to one or more leadwires of implantable therapy delivery leads. These implantable therapy delivery leads can effectively act as antennas of electromagnetic interference (EMI) signals. Therefore, when these electromagnetic signals enter within the interior space of a hermetic implantable medical device, facilitated by the therapy delivery leads, they can negatively impact the intended function of the medical device and as a result, negatively impact therapy delivery intended for a patient by that device. EMI engineers commonly refer to this as the “genie in the bottle” effect. In other words, once the genie (i.e., EMI) is inside the hermetic housing of the device, it can wreak havoc with electronic circuit functions by cross-coupling and re-radiating within the device.
Another particularly problematic condition associated with implanted therapy delivery leads occurs when a patient is in an MRI environment. In this case, the electrical currents imposed on the implanted therapy delivery leads can cause the leads to heat to the point where tissue damage is likely. Moreover, RF currents (electromagnetic interference—EMI) may be coupled to implanted therapy delivery leads resulting in undesirable electrical currents which can enter the AIMD and can disrupt or damage the sensitive electronics within the implantable medical device.
Therefore, materials selection and fabrication processing parameters are of utmost importance in creating a hermetic terminal (or feedthrough) or a structure embodying a hermetic terminal (or feedthrough), that can survive anticipated and possibly catastrophically damaging environmental conditions and that can be practically and cost effectively manufactured.
Hermetic terminals or feedthrough assemblies utilizing ceramic dielectric materials may fail in a brittle manner. A brittle failure typically occurs when the ceramic structure is deformed elastically up to an intolerable stress, at which point the ceramic fails catastrophically. Most brittle failures occur by crack propagation in a tensile stress field. Even microcracking caused by sufficiently high tensile stress concentrations may result in a catastrophic failure including loss of hermeticity identified as critical in hermetic terminals for implantable medical devices. Loss of hermeticity may be a result of design aspects such as a sharp corner which creates a stress riser, mating materials with a difference of coefficient of thermal expansions (CTE) that generate tensile stresses that ultimately result in loss of hermeticity of the feedthrough or interconnect structure.
In the specific case of hermetic terminal or feedthrough designs, a tensile stress limit for a given ceramic based hermetic design structure cannot be specified because failure stress in these structures is not a constant. As indicated above, variables affecting stress levels include the design itself, the materials selection, symmetry of the feedthrough, and the bonding characteristics of mating surfaces within the feedthrough. Hence, length, width and height of the overall ceramic structure matters as do the number, spacing, length and diameter of the conductive pathways (vias, terminal pins, leadwires, etc.) in that structure. The selection of the mating materials, that is, the material that fills the vias (or leadwire) and the material that forms the base ceramic, are important. Finally, the fabrication processing parameters, particularly at binder burnout, sintering and cool down, make a difference. When high reliability is required in an application such as indicated with hermetic terminals or feedthroughs for AIMDs, to provide insurance for a very low probability of failure it is necessary to design a hermetic terminal assembly or feedthrough structure so that stresses imparted by design, materials and/or processing are limited to a smaller level of an average possible failure stress. Further, to provide insurance for a very low probability of failure in a critical ceramic based assembly or subassembly having sustainable hermetic requirements, it is also necessary to design structures embodying a hermetic terminal or feedthrough such that stresses in the final assembly or subassembly are limited to a smaller level of an average possible failure stress for the entire assembly or subassembly. In hermetic terminals and structures comprising hermetic terminals for AIMDs wherein the demand for biocompatibility exists, this task becomes even more difficult.
The most critical feature of a feedthrough design or any terminal subassembly is the metal/ceramic interface within the feedthrough that establishes the hermetic seal. One embodiment of the present invention therefore provides where a hermetic feedthrough comprising a monolithic alumina insulator substrate within which a platinum conductive pathway or via resides or wherein a metallic leadwire (terminal pin) resides. More specifically in the case of a filled via, the present invention provides a hermetic feedthrough in which the hermetic seal is created through the intimate bonding of a platinum metal residing within the alumina substrate.
A traditional ceramic-to-metal hermetic terminal is an assembly of three components: electrical conductors (leadwires, pins, terminal pins, filled vias) that conduct electrical current, a ceramic insulator, and a metal housing, which is referred to as the flange or the ferrule. Brazed joints typically hermetically seal the metal leadwires and the flange or ferrule to the ceramic insulator. For a braze-bonded joint, the braze material is generally intended to deform in a ductile manner in order to compensate for perturbations that stress the bond between the mating materials as the braze material may provide ductile strain relief when the thermal expansion mismatch between the ceramic and metal is large. Thus, mating materials with large mismatches in CTE can be coupled through braze materials whose high creep rate and low yield strength reduce the stresses generated by the differential contraction existing between these mating materials.
Thermal expansion of metal is generally considerably greater than those of ceramics. Hence, successfully creating a hermetic structure, and one that can sustain its hermeticity in service, is challenging due to the level of residual stresses in the final structure. Specifically, thermal expansion mismatch results in stresses acting along the ceramic/metal interface that tend to separate the ceramic from the metal and so the bond developed between the ceramic and the metal must be of sufficient strength to withstand these stresses, otherwise adherence failure, that is, loss of hermeticity, will occur. One method for limiting these stresses is to select combinations of materials whose thermal contractions after bonding are matched.
In making the selection for a CTE match, it is important to note that very few pairs of materials have essentially identical thermal expansion curves. Generally, the metal component is selected first based on electrical and thermal conductivity, thermal expansion, ability to be welded or soldered, mechanical strength, and chemical resistance or biocompatibility requirements. The ceramic is then selected based primarily on electrical resistivity, dielectric strength, low gas permeability, environmental stability, and thermal expansion characteristics. In the specific case of selecting platinum wire, often the ceramic formulation is modified in order to match its CTE to that of the platinum wire. In yet a more specific case of selecting platinum paste, the platinum paste formulation may be modified as well. If the mating materials are alumina of at least 96% purity and essentially pure platinum paste, then matching CTE is not possible. Thus, for AIMD's, consistently achieving hermetic terminal structures that are capable of sustaining hermeticity throughout the application's service life has proven challenging. Another solution would be to use a cermet which is part ceramic and part metal. For example, the cermet could be an alumina/platinum paste that then had a closer CTE to that of the alumina insulator.
Producing a stress-free structure often not only involves bonding a pair of materials but also achieving that bond at a very specific temperature so that their contractions on cooling to room temperature are essentially the same even though the contraction curves may not coincide. Since this often is a significant challenge, hermetic terminals are produced by metalizing the alumina and using a brazing material to form the bond at some other temperature than an intersection of the CTE curves. (NOTE: Forming a bond between two materials that become rigid at the intersection of the two CTE curves makes it possible to produce a structure that is stress free at room temperature, unless the two CTE curves separate substantially from each other from the intersection point and room temperature.) The deformation of the braze material by time-independent plastic flow or creep relaxation limits the stresses generated in the ceramic. Given this, the impact of the rate of cooling on the final stress level of a structure must also be considered. In some cases, residual stresses are generated deliberately to provide protective compressive stresses in the ceramic part and in the bond interface. Usually this is accomplished by selecting components with different CTEs. Another way is to control the shrinkage of one material over its mating material. In either case, it is important to minimize stress levels such that the interface on which hermeticity depends is well within the stress level at which failure might occur.
In an embodiment, the present invention is directed to mating bound particulate high purity alumina of at least 96% and particles of essentially pure platinum metal that are suspended within a mixture of solvents and binders, i.e., a platinum paste. This combination of materials does not use a braze material to buffer the CTE mismatch between these two materials. Further, since the intent of this embodiment is to provide hermetic terminals and subassemblies comprising hermetic terminals for AIMDs, this particularly embodiment does not consider modifications to the alumina formulation or the platinum paste in an attempt to match their CTEs. Rather, this embodiment discloses sustainable hermetic terminals and structures embodying these hermetic terminals. This is achieved by adjusting platinum paste solids loading, prescribing via packing, prescribing binder burnout, sintering and cool down parameters, such that shrinkage of the alumina is greater than the shrinkage of the platinum fill in the via and an intimate and tortuous (a mutually conformal) interface is created that may be either a direct bond between the alumina and platinum materials that is hermetic, or alternatively, that may develop an amorphous interfacial layer that is not susceptible to erosion by body fluids and can tolerate stress levels without losing hermeticity. (It will be understood by those skilled in the art, that this teaching contains other embodiments that are not dependent upon using an essentially pure platinum paste as a conductive fill. Furthermore, many of the embodiments presented herein don't use a conductive filled via but rather a leadwire.)
Regarding EMI, a terminal or feedthrough capacitor EMI filter may be disposed at, near or within a hermetic terminal or feedthrough resulting in a feedthrough filter capacitor which diverts high frequency electrical signals from lead conductors to the housing or case of an AIMD. Many different insulator structures and related mounting methods are known in the art for use of feedthrough capacitor EMI filters in AIMDs, wherein the insulative structure also provides a hermetic terminal or feedthrough to prevent entry of body fluids into the housing of an AIMD. In the prior art devices, the hermetic terminal subassembly has been combined in various ways with a ceramic feedthrough filter EMI capacitor to decouple interference signals to the housing of the medical device.
In a typical prior art unipolar construction (as described in U.S. Pat. No. 5,333,095 and herein incorporated by reference), a round/discoidal (or rectangular) ceramic feedthrough EMI filter capacitor is combined with a hermetic terminal pin assembly to suppress and decouple undesired interference or noise transmission along a terminal pin. The feedthrough capacitor is coaxial having two sets of electrode plates embedded in spaced relation within an insulative dielectric substrate or base, formed typically as a ceramic monolithic structure. One set of the electrode plates are electrically connected at an inner diameter cylindrical surface of the coaxial capacitor structure to the conductive terminal pin utilized to pass the desired electrical signal or signals. The other or second set of electrode plates are coupled at an outer diameter surface of the round/discoidal capacitor to a cylindrical ferrule of conductive material, wherein the ferrule is electrically connected in turn to the conductive housing of the electronic device. The number and dielectric thickness spacing of the electrode plate sets varies in accordance with the capacitance value and the voltage rating of the coaxial capacitor. The outer feedthrough capacitor electrode plate sets (or “ground” plates) are coupled in parallel together by a metalized layer which is fired, sputtered or plated onto the ceramic capacitor. This metalized band, in turn, is coupled to the ferrule by conductive adhesive, soldering, brazing, welding, or the like. The inner feedthrough capacitor electrode plate sets (or “active” plates) are coupled in parallel together by a metalized layer which is either glass frit fired or plated onto the ceramic capacitor. This metalized band, in turn, is mechanically and electrically coupled to the lead wire(s) by conductive adhesive, soldering, or the like. In operation, the coaxial capacitor permits passage of relatively low frequency biologic signals along the terminal pin, while shielding and decoupling/attenuating undesired interference signals of typically high frequency to the AIMD conductive housing. Feedthrough capacitors of this general type are available in unipolar (one), bipolar (two), tripolar (three), quad polar (four), pentapolar (five), hexpolar (6) and additional lead configurations. The feedthrough capacitors (in both discoidal and rectangular configurations) of this general type are commonly employed in implantable cardiac pacemakers and defibrillators and the like, wherein the pacemaker housing is constructed from a biocompatible metal such as titanium alloy, which is electrically and mechanically coupled to the ferrule of the hermetic terminal pin assembly which is in turn electrically coupled to the coaxial feedthrough filter capacitor. As a result, the filter capacitor and terminal pin assembly prevents entrance of interference signals to the interior of the pacemaker housing, wherein such interference signals could otherwise adversely affect the desired cardiac pacing or defibrillation function.
Referring once again to feedthrough capacitor EMI filter assemblies, although these assemblies as described earlier have performed in a generally satisfactory manner, and notwithstanding that the associated manufacturing and assembly costs are unacceptably high in that the choice of the dielectric material for the capacitor has significant impacts on cost and final performance of the feedthrough filter capacitor, alumina ceramic has not been used in the past as the dielectric material for AIMD feedthrough capacitors. Alumina ceramic is structurally strong and biocompatible with body fluids but has a dielectric constant around 6 (less than 10). There are other more effective dielectric materials available for use in feedthrough filter capacitor designs. Relatively high dielectric constant materials (for example, barium titivate with a dielectric constant of over 2,000) are traditionally used to manufacture AIMD feedthrough capacitors for integrated ceramic capacitors and hermetic seals resulting in more effective capacitor designs. Yet ceramic dielectric materials such as barium titinate are not as strong as the alumina ceramic typically used to manufacture the hermetic seal subassembly in the prior art. Barium titinate is also not biocompatible with body fluids. Direct assembly of the ceramic capacitor can result in intolerable stress levels to the capacitor due to the mismatch in coefficients of thermal expansion between the titanium pacemaker housing (or other metallic structures) and the capacitor dielectric. Hence, particular care must be used to avoid cracking of the capacitor element. Accordingly, the use of dielectric materials with a low dielectric constant and a relatively high modulus of toughness are desirable yet still difficult to achieve for capacitance-efficient designs.
Therefore, it is very common in the prior art to construct a hermetic terminal subassembly with a feedthrough capacitor attached near the inside of the AIMD housing on the device side. The feedthrough capacitor does not have to be made from biocompatible materials because it is located on the device side inside the AIMD housing. The hermetic terminal subassembly includes conductive pathways (leadwires, pins, terminal pins, filled vias, etc.) to hermetically pass through the insulator in non-conductive relation with the ferrule or the AIMD housing. The conductive pathways also pass through the feedthrough hole of the capacitor to electronic circuits disposed inside of the AIMD housing. These leadwires are typically electrically continuous and, on the body fluid side, must be biocompatible and non-toxic. Generally, these conductive pathways are constructed of platinum or platinum-iridium, palladium or palladium-iridium, niobium pins or filled vias with conductive powders, ceramics, gradient materials or the like. Platinum-iridium is an ideal choice because it is biocompatible, non-toxic and is also mechanically very strong. The iridium is added to enhance material stiffness and to enable the hermetic terminal subassembly leadwire to sustain bending stresses. An issue with the use of platinum for leadwires is that platinum has become extremely expensive and may be subject to premature fracture under rigorous processing such as ultrasonic cleaning or application use/misuse, possibly unintentional damaging forces resulting from Twiddler's Syndrome. Twiddler's Syndrome is a situation documented in the literature where a patient will unconsciously or knowingly twist the implantable device to the point where attached leads may even fracture.
Accordingly, what is needed is a filtered structure like a hermetic terminal or feedthrough, any subassembly made using same and any feedthrough filter EMI capacitor assembly which minimizes intolerable stress levels, allows use of preferred materials for AIMDs or eliminates high-priced, platinum, platinum-iridium or equivalent noble metal hermetic terminal subassembly leadwires. Also, what may be needed is an efficient, simple and robust way to connect the leadwires in a header block to the novel hermetic terminal subassembly. Correspondingly, it is also needed to make a similar efficient, simple and robust electrical connection between the electronics on the device side of the AIMD to the feedthrough capacitor and hermetic terminal subassembly. The present invention fulfills these needs and provides other related advantages.